Serum IgE and IgG antibody activity against Aspergillus fumigatus was measured in 3 groups of subjects by 2 different immunologic methods. Group A consisted of 23 patients with allergic bronchopulmonary aspergillosis (ABPA). Group B was composed of 19 patients with extrinsic asthma who had marked immediate type skin reactivity to A. fumigatus (prick skin test, 3 or 4+) but no other manifestation of ABPA. Group C, the control group, was composed of 12 healthy subjects. Two immunological methods, including a solid-phase polystyrene tube radioimmunoassay and an iodine-125-labeled, A. fumigatus antigen radioimmunoassay, were used to study each patient's serum sample, so as to demonstrate IgE antibody activity against A. fumigatus (IgE-Af) and IgG antibody activity against A. fumigatus (IgG-Af). Both IgE-Af and IgG-Af were significantly greater among patients in Group A than among those in Group B and Group C, as measured by both methods (P is less than 0.001). The results of this study suggest that either method can be used as a diagnostic aid for ABPA. These methods may provide a laboratory test permitting diagnosis of ABPA in its early stages before bronchial or pulmonary destruction occurs.